Conventionally, a thin-film semiconductor element called a thin-film transistor (TFT) is used in an active matrix display device such as a liquid crystal display device or an organic electro luminescence (EL) display device, or a solid-state imaging device such as a digital camera.
In the active matrix display device (display panel), the TFT is used as a switching transistor for selecting a pixel, a driving transistor for driving the pixel, a transistor of a driver outside the panel, or the like.
For example, unlike a voltage-driven liquid crystal display, an organic EL display having an organic EL element utilizing an EL which is an organic material is a current-driven display device, so that development of the TFT having superior performance is immediately needed.
The TFT has a gate electrode, a semiconductor layer (channel layer), a source electrode, and a drain electrode formed above a substrate. Generally, an amorphous silicon thin film or a polysilicon thin film is used for the channel layer.
In view of easiness in manufacturing, an amorphous silicon TFT in which the amorphous silicon thin film is used as the channel layer generally has so-called a bottom gate structure, in which the gate electrode is below the channel layer.
On the other hand, in order to demonstrate the performance at a maximum, a polysilicon TFT in which the polysilicon thin film is used for the channel layer generally has so-called a top gate structure, in which the gate electrode is above the channel layer.
Recent years have seen more development of TFT using an oxide semiconductor represented by In—Ga—Zn—O (IGZO) for the channel layer. As a structure of the TFT using the oxide semiconductor, the bottom gate structure, which is the same as the structure of the conventional amorphous silicon TFT, is common. Research and development is also carried out for the top gate structure having higher performance which is capable of reducing parasitic capacitance between the gate electrode and the source electrode or between the gate electrode and the drain electrode (for example, Patent Literature (PTL) 1 and 2).